PROJECT SUMMARY The post-thrombotic syndrome (PTS) is a permanent complication of lower extremity deep vein thrombosis (DVT). Patients with moderate-to-severe PTS secondary to chronic iliac vein obstruction experience profound disability and quality of life impairment from pain, massive edema, stasis dermatitis, and/or venous leg ulcers (Disabling Iliac-Obstructive PTS [DIO-PTS]). Currently, there is no evidence-based therapy for DIO-PTS. Observational studies and a small single-center randomized trial suggest that imaging-guided, catheter-based endovascular therapy (EVT) that eliminates iliac vein obstruction (stent placement) and saphenous venous valvular reflux (endovenous ablation) is effective. However, EVT has risks and is costly. We therefore plan to do an open-label, assessor-blinded, multicenter, randomized controlled trial (the Chronic Venous Thrombosis: Relief with Adjunctive Catheter-Directed Therapy (C-TRACT) Study) that will compare EVT with No-EVT in 374 patients with DIO-PTS. All patients will receive optimal standardized medical therapy for PTS. The primary outcome will be within-subject improvement in PTS severity over 6 months, assessed blindly using the Venous Clinical Severity Score (VCSS). Secondary outcomes include PTS severity (VCSS, Villalta, ulcer healing), and venous disease-specific (VEINES-QOL) and generic (SF-36) quality of life at 6 and 24 months. A comprehensive health economic analysis will compare medical costs and estimate the incremental cost-effectiveness of EVT. Safety (e.g. bleeding, recurrent thrombosis) will be assessed at 2, 6, and 24 months, and venous imaging will be performed to identify anatomic and physiologic predictors of therapeutic response. The C-TRACT Study will change clinical practice: if EVT is effective and safe, it will become part of standard therapy for DIO-PTS; if not, a risky and costly therapy will be avoided. Hence, either study outcome will improve public health and advance the NHLBI?s mission.